An American study has uncovered a shift in teenagers' consumption and perceptions of cannabis since the 2015 legalization of recreational use of the drug in the states of Washington.

The data showed that legalization of recreational marijuana use significantly reduced perceptions of marijuana's harmfulness by 14 percent and 16 percent among eighth and 10th graders and increased their past-month marijuana use by 2 percent and 4 percent in Washington state but not in Colorado.

"While legalization for recreational purposes is now limited to adults, potential impacts on adolescent marijuana use are of particular concern", said Magdalena Cerdá, an epidemiologist with the UC Davis Violence Prevention Research Program and first author of the study.

They join Colorado, which legalized in 2012, Oregon, Alaska and the State of Washington, which legalized in 2015, as well as the U.S. capital Washington DC, which legalized in 2014.

The researchers compared data on the perceived harmfulness of marijuana use to health and self-reported marijuana use for almost 254,000 Colorado and Washington state students in the eighth, 10th and 12th grades who participated in the Monitoring the Future survey.

About 61 percent of eighth-graders and 47 percent of 10th-graders in Washington saw marijuana as greatly or moderately risky to health in 2013-2015, compared with 75 percent and 63 percent in 2010-2012, the findings showed.

Among states without legalizedmarijuana use, the perceived harmfulness also decreased by 5 percent and 7 percent for students in the two grades, but marijuana use decreased by 1.3 percent and.9 percent. According to a recent study, adolescents in the eighth and 10th grade are less concerned about the negative effects of the use of marijuana.

No changes were seen in perceived harmfulness or marijuana use among Washington 12th-grades or students in the three grades in Colorado, for which researchers offer several explanations in their article.

As the debate continues in many states over whether legal marijuana is bad for society, in at least one crucial area the evidence seems to be in regarding the medicinal form: Legal medical pot appears to be saving lives on our highways. Astudy published just this month in Cancer Medicine found marijuana use to be linked to a lower risk of heart failure, cardiac disease, and in-hospital mortality.

Under the initiative adults above 21 would be allowed to possess and use limited amounts of recreational marijuana and grow as many as a dozen potted plants in their homes.

While research into marijuana and its effects are still hard to come by because of federal restrictions, this year several key studies were published indicating cannabis as safe and therapeutically beneficial.

Do you believe that marijuana legalization will lead to more teen pot use? But at the same time, adolescent use of marijuana has been flat or falling.

The authors of the JAMA Pediatrics study said that, given the findings, states that legalizemarijuana should also invest in substance-abuse-prevention programs for teens.